Coiled heaters for indirectly heated cathodes



Dec. 17, 1963 HIROSHI YAMAMOTO ETAL 3,114,356

COILED HEATERS FOR INDIRECTLY HEATED CATHODES Filed March 9, 1962 Fig Fig. 2a

PRIOR/1R1 United States Patent Ofitice Patented Dec. 17, 1963 3,114,856 COILED HEATERS FOR INDIRECTLY HEATED CATHODES Hiroshi Yamamoto, Hodogaya-ku, Yokohama, and

Yukio Takanashi, Hiratsuka-shi, Japan, assignors to Tokyo Shibaura Electric Co., Ltd., Kawasaki-ski,

Japan, a corporation of Japan Filed Mar. 9, 1962, Ser. No. 178,6tl7 Claims priority, application Japan Mar. 13, 1961 2 Claims. (Cl. 313-340) The present invention relates to improvements in or relating to coiled heaters for use in indirectly heated type electron tubes and has for its primary object to provide an improved coiled heater of the kind described which is hardly subject to wire breakage.

An additional object of the present invention is to provide an improved coiled heater which has a reduced heat capacity enabling the cathode-ray tube employing such coiled heater to start rapidly compared with one employing a conventional coiled heater.

According to the present invention, there is provided a coiled heater for use in an indirectly heated cathode comprising a thin wire of high-melting metal wound into a helical coil of elliptical cross section and an insulating coating deposited on the peripheral surface of the coil.

These and other objects, features and advantages of the present invention will be apparent from the following description when read with reference to the accompanying drawings, in which:

FIG. 1 is an enlarged side view, partly in section, of a conventional coiled heater;

FIG. 2a is a side elevation of a cathode cup with the conventional heater fitted therein;

FIG. 2b is an enlarged cross sectional view taken along the line A-A in FIG. 20;

FIG. 3a is an enlarged side view, partly in section, of a heater constructed according to the present invention;

FIG. 3b is a cross sectional view taken along the line AA in FIG. 3a; and

FIG. 4 is an enlarged transverse cross section illustrating the step of coiling the heater wire.

For instance, in a cathode-ray tube employed in a transistorized portable television receiving set, a heater of extremely limited power input, for example having ratings of 12.6 volts and 75 milliamperes, is employed for economy in battery power consumption.

Such heater has a core wire which is necessarily extremely thin and large in length. For example, a core wire 1 of high-melting metal such as tungsten and having a thickness of approximately 0.029 millimeter and a length of approximately 280 millimeters is wound into a coil of approximately 0.18 millimeter outside diameter and approximately 22 millimeters length. The coil is bent into an M-shape and an insulating coating 2 of aluminum oxide is applied around the bent coil in a thickness of approximately 0.07 millimeter to form a heater (FIG. 1). The heater constructed in this manner has a more or less flexibility and is inserted for example in a nickel cathode cup 3 of approximately 0.09 millimeter inside diameter and approximately 6 millimeters length to form a heater cathode assembly. In this case, when the heater is energized, the cathode cup is elongated to a much greater extent than the heater because of the higher coefficient of thermal expansion of the nickel. On the other hand, with a heater-cathode assembly dimensioned as described above, the side of the heater is substantially in pressure contact with the inner wall of the cathode cup and thus the elongation of the latter develops a substantial tensioning stress in the heater so that the heater core wire, formed of a very thin metal Wire as described above, is frequently broken as the heater is repeatedly switched on and off.

According to the present invention, an improved coiled heater is provided by forming a core wire into a helical coil of elliptical or oblong cross section thereby substantially to reduce any tensioning stress occurring in the wire material for the purpose of eitectively preventing wire breakage. FIGS. 3a and 3!) illustrate one example actually made by the invent-or. The core wire 1 of tungsten having a thickness of approximately 0.029 millimeter and a length of approximately 280 millimeters was wound into an elliptical coil having an outer minor diameter of approximately 0.236 millimeter, an outer major diameter of approximately 0.414 millimeter and a length of approximately 12 millimeters. The coil was then bent into a V-shape in the plane including the minor diameter of the coil section. An insulator coating 2 of approximately 0.07 millimeter thickness and consisting of aluminum oxide was deposited on the periphery of the bent coil.

The heater thus formed according to the present invention exhibited a more or less flexibility, which was sufiicient to enable the heater to be fitted into a cathode cup dimensioned as described. The heater-cathode assembly thus formed was employed to produce a cathode-ray tube, which was subjected to repeated on-off tests proving its extraordinary resistance to wire breakage.

As will readily be understood, this advantageous characteristic derived from the fact that the length. of the core wire per unit length of the coil was increased to a substantial extent so that the tensioning stress applied to the core wire owing to greater thermal expansion of the cathode cup was actually greatly reduced. Such coil of elliptic cross section may be formed by winding a core wire about a metal wire having a desired cross-sectional configuration, or alternatively may be formed simply by winding the core wire around two juxtaposed metal wires each of circular cross section, as shown in FIG. 4.

Additionally, according to the present invention, the amount of the coating material as deposited on the heater coil is reduced by approximately twenty percent compared with that on a heater coil of conventional make, substan tially reducing the heat capacity of the heater according to the present invention. This enables the cathode to be heated to a desired temperature with rapidity, resulting in an additional advantage that the time length required for a picture to be developed on the cathode-ray tube is reduced approximately ten percent.

In the past there have been contemplated a number of methods of inserting a heater core of substantial length into a cathode cup of limited diameter and length as described. In one such method, the core wire is folded back a number of times, and in another it is wound into a double helical coil. The former method involves a deficiency in connection with the electric characteristics of the heater obtained, while the latter method is practically infeasible with such thin core wires as described because of limitations to the coil manufacturing techniques. A further method is to wind the core wire into a coil of circular cross section and having a limited coil length and then to bend the coil thus formed into a V- shape, which is to be inserted into a cathode cup. However, the coil formed of a core wire of such great length has a substantially large diameter so that it cannot be properly inserted into a cathode cup as described above.

The present invention has solved the above difliculties previously met with success by forming a coil in elliptical cross section and in correspondingly increased diameter. As will readily be appreciated, the present invention is quite useful in the manufacture of general electronic tubes including cathode-ray tubes.

What is claimed is:

11. A heater for use in an indirectly-heated cathode comprising a wire wound into a helical coil; said coil being folded into a substantially V-shaped configuration; and an insulating layer covering the Wire and coil.

2. An indirectly-heated cathode for an electrical discharge tube, comprising a cylindrical cathode cup, and a heater element fitting in said cathode cup as a Whole, said heater element comprising a thin bare wire of highmelting metal wound into a helical coil having elliptical cross-sections with substantially uniform major and minor diameters throughout the Whole length of said coil, said helical coil as a whole being twice-folded in the lane including therein the minor diameters of said elliptical A cross-sections, and an insulating layer of substantially uniform thickness coating said helical coil as a whole, said coating layer being applied around said coil after the latter has been twice-folded.

References Cited in the file of this patent UNITED STATES PATENTS 

2. AN INDIRECTLY-HEATED CATHODE FOR AN ELECTRICAL DISCHARGE TUBE, COMPRISING A CYLINDRICAL CATHODE CUP, AND A HEATER ELEMENT FITTING IN SAID CATHODE CUP AS A WHOLE, SAID HEATER ELEMENT COMPRISING A THIN BARE WIRE OF HIGHMELTING METAL WOUND INTO A HELICAL COIL HAVING ELLIPTICAL CROSS-SECTIONS WITH SUBSTANTIALLY UNIFORM MAJOR AND MINOR DIAMETERS THROUGHOUT THE WHOLE LENGTH OF SAID COIL, SAID HELICAL COIL AS A WHOLE BEING TWICE-FOLDED IN THE PLANE INCLUDING THEREIN THE MINOR DIAMETERS OF SAID ELLIPTICAL CROSS-SECTIONS, AND AN INSULATING LAYER OF SUBSTANTIALLY UNIFORM THICKNESS COATING SAID HELICAL COIL AS A WHOLE, SAID COATING LAYER BEING APPLIED AROUND SAID COIL AFTER THE LATTER HAS BEEN TWICE-FOLDED. 